Various chemically reactive coating compounds which are characterized by the occurrence of a chemical reaction during the mixing of the compounds are known in the art. A number of these chemically reactive materials are multi-component systems typically involving two or more materials which react upon mixing. Representative multi-component systems include isocyanate/hydroxyl, anhydride/hydroxyl, aminelepoxy and other reactive systems well known in the art. In a clearcoat system comprising a hydroxy functional polymer as the first component and a polyisocyanate as a second component, for example, the two coating components must be mixed prior to their being supplied to a spray gun for application onto a desired surface such as an automobile body. If the mixed materials have a sufficiently long pot life, they can be mixed in a container and applied in a conventional manner. If the materials react very quickly, as is frequently desirable, however, the usable time period after mixing may be very short, and the materials must be mixed immediately prior to application. In such a case, generally the two coating components mix in a T-shaped mixing chamber which the two coating components enter separately, through coating component inlets, on opposite sides of the top of the T. They mix when they meet in a reservoir at the top of the T and as they join and flow out of the bottom of the T into a single supply line which leads to the spray gun. The spray gun applies the mixed material through the use of compressed air to atomize the mixed material.
In addition to the two coating component inlets mentioned above, most T-shaped mixing chambers have a third inlet, for solvent, which is generally placed at the bottom of the T. T-shaped mixing blocks normally permit a user to select between a mixing function and a cleaning function by manually shifting of coating component valves and a solvent valve. In order to switch from a multi-component coating system to solvent a user first rotates both coating component valve levers at the mixing block ninety degrees (90.degree.) in order to shut off the flow of coating components to the T-shaped mixing block. In order to accomplish this a user usually has to put down the spray gun in order to manipulate the valve levers. Then to start solvent to flowing into the T-shaped mixing block a user must switch over the pump on the supply station from supplying coating components to supplying solvent. This is generally accomplished by toggling a pneumatic switch (when the pump is air driven) which shuts off air to the coating component pumps and turns on air to the solvent pump. As a result, a user is required to actuate two sets of valves at different locations in order to accomplish the task of cleaning the conventional T-shaped mixing block. Such a task is both awkward and time consuming.
As discussed above, the solvent necessary for cleaning is supplied to a T-shaped mixing block via a solvent inlet which is connected to a solvent supply line. Since the inlet for solvent is usually at the bottom of the T-shaped mixing chamber of a T-shaped mixing block, generally only the T-shaped mixing chamber is throughly cleaned, and coating component inlets and their control valves may not be thoroughly cleaned. This may result in the buildup of reactive coating components or contamination between different multi-component systems.
In addition, if a T-shaped mixing block is removed from the coating component and solvent supply lines, there is a considerable amount of waste generated by coating components and solvent which remain in the supply lines. Normally, purging all of the hoses in a system utilizing a T-shaped mixing block as described above can generate approximately 1600 ml of total waste in a system with 50 feet of hose (per supply line). Purging the supply lines increases the cost to the user of such a system. Furthermore, such multi-component coating systems and the solvents used to clean the equipment are usually treated as hazardous wastes, so minimizing waste reduces disposal costs.
Furthermore, a T-shaped mixing block may suffer from sputtering and/or pump pulsation problems at low pump pressures due to a cycling effect. The cycling of the pump in the supply station may cause a fluctuation in pump pressures which may lead to uneven application of a mixed material by a user.
In addition, a T-shaped mixing block usually has to be mounted on a spray gun for support. Positioning a T-shaped mixing block on a spray gun restricts the range of motion of the spray gun. Such a restriction in the motion of a spray gun may require a user to utilize awkward positions in order, for example, to paint wheel wells or under panels on automobiles.
Consequently, there is a strong need in the art for a mixing block which: maximizes the mixing of coating components of a multi-component system, permits the automatic cleaning of the inlet valves by the solvent, is effective in low pressure applications by preventing sputtering and/or pump pulsation, permits placement of the mixing block a significant distance away from the spray gun.